RAREST PARTICLE DECAY
SEEN AT BROOKHAVEN LAB

UPTON, NY -- After ten years of searching, an international collaboration
of physicists working at the U.S. Department of Energy's Brookhaven National Laboratory
(BNL) believes it has seen the rarest decay of a subatomic particle ever
detected.

The phenomenon is thought to happen only once or
twice in every 10 billion self-destructions of an unstable particle known
as a kaon. Instead of producing the usual breakdown products seen when a
kaon decays, the rare kaon decay, as it is called, is thought to have released
a positively charged pi meson, a neutrino and an anti-neutrino.

Not only is the proccess rare, it is also extremely
elusive. The scientists report that to see even one such event using the
most sensitive equipment at BNL's Alternating
Gradient Synchrotron accelerator, they had to
sift through one trillion ordinary decays to achieve the one-in-ten-billion
level of sensitivity required.

The spotting of the rare kaon decay sheds new light
on the universe's most elemental forces and most basic building blocks,
as explained by the extraordinarily successful theory of subatomic particles
known as the Standard Model. It may also suggest new phenomena that cannot
be explained by the Standard Model.

Rare, But Important

Because of its highly unusual nature, the knowledge
gained in studying this decay is expected to be exceptionally important
to particle physicists.

"This is a phenomenon that physicists have been looking for since the
1960s, but that nobody knew for sure we would see," said Douglas Bryman
of TRIUMF, one of three co-spokesmen for the collaboration, which is known
as E787. "Now, after years of searching, we believe we have
seen it."

Added co-spokesman and Brookhaven physicist Laurence
Littenberg, "From here, it is up to us and others to test that belief
through further exploration and experimentation. We plan to collect and
analyze ten times more data in order to gauge its consistency with the Standard
Model, and to test the possibility that the event we've seen could even
involve entirely new particles or forces."

The usual decays (and similar interactions) seen in particles and in radioactive
atomic nuclei occur by the transmission of one massive W or Z boson, which
are carriers of the weak force in the same way that the particle form of
light, known as photons, carries the electromagnetic force.

The Standard Model predicts that the decay of a
kaon to a pi meson and a neutrino pair sometimes involves the momentary
creation of both a charged W boson and a neutral Z boson (which itself instantly
decays into the two neutrinos), rather than the more easily produced exchange
of a single W or Z. It can also involve the recently discovered massive
top quark, and thus give a window into the relation between that exotic
object and the normal quarks which make up our everyday world.

Understanding such complex forms of decay is especially
important to physicists attempting to learn how matter behaves at the most
fundamental level. The one-in-ten-billion probability of a kaon decaying
to a pi meson and a neutrino pair is a remarkable prediction of the Standard
Model and one that the experimenters set out to test.

Catching A Shooting Star

Finding the rare kaon decay required an accelerator
powerful enough to produce kaons in vast numbers, making BNL's AGS an appropriate
choice. Recent upgrades there made the accelerator capable of producing
the world's most intense kaon beam.

But equally important was the team's array of detectors
sensitive enough to catch the particle equivalent of a shooting star: Kaons
last only about 12 billionths of a second before decaying, and they can
decay a multitude of different ways, creating showers of particles that
can only be seen with specialized equipment.

So, to catch a fleeting pi meson, the E787 team,
led by Bryman, Littenberg and A.J. Stewart Smith of Princeton, in 1995 built
a new "catcher's mitt," located in a strong magnetic field and
made up of sophisticated particle detectors used to measure as much as possible
about each pi meson that passed by. These detectors included scintillating
fibers, a tracking chamber and several other devices used to determine the
energy and momentum of the pi meson and to observe its characteristic decay
into other particles.

The improved equipment increased the chances of
seeing a rare decay if it occurred, and vastly reduced the chances of confusing
it with other phenomena that send out nearly the same signal but happen
billions of times more often.

"The experiment's state-of-the-art apparatus
is sensitive enough to examine one million decays per second," said
Littenberg. "We collected thousands of gigabytes of data,and
out of all that data, we saw one event that was completely unexplainable
except by the rare kaon decay we were searching for."

A Nod to the Past, a Glimpse of
the Future

The discovery ties in to both past and future research at BNL's accelerators.

"It's especially fitting we should see this
phenomenon at the AGS, since kaon decays have figured centrally in past
important discoveries there," said Bryman. "The most notable example,
of course, is the work on CP-violation that won the 1980
Nobel Prize for James Cronin and Val Fitch. That
discovery, which among other things may help explain why there's more matter
than antimatter in the universe, was an unanticipated rare kaon reaction
which had a revolutionary impact."

And, he added, the E787 collaboration plans to
continue the present study of rare kaon decays for the next several years,
even after the AGS becomes the injector for the Relativistic
Heavy Ion Collider (RHIC)when it begins operations
in 1999.

There are also plans to study the closely related
decay of the long-lived neutral kaon into a neutral pi meson and a pair
of neutrinos, a process that may offer the single best window into the still-mysterious
phenomenon of CP-violation.

-- 30 --

RARE KAON DECAY GRAPHICS

A pi meson travels swiftly through the detectors in the E787 experiment
at BNL's Alternating Gradient Synchrotron after another particle, called
a kaon (red striped squares), spontaneously decays. The pi emerges from
the stopping target (its path is shown by the blue squares) and enters the
detector's central drift chamber (the small circles in the drift chamber
are tangent to the path of the pion). It then penetrates the range stack,
an array of scintillators (excited ones shown as blue rectangles) and chambers
and loses energy until it stops. The signal in the scintillator where it
stops is shown at the top right. The double pulse is characteristic of a
pion. After examining 1.5 trillion events, the E787 collaboration found
the pictured decay in which a pion is completely unaccompanied by other
detectable particles, an important find for physics.

Members of the E787 collaboration at Brookhaven
National Laboratory's Alternating Gradient Synchrotron pose in front of
their apparatus, which allowed them to spot the rarest particle phenomenon
ever seen.